The 3rd Generation Partnership Project (3GPP) is responsible for standardization of the Universal Mobile Telecommunication Service (UMTS) system and Long Term Evolution (LTE). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink directions, and is thought of as a next generation mobile communication system of the UMTS system. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). The first release of LTE, also referred to as 3GPP Release 8 or Rel-8 can provide peak rates of 300 Mbps, a radio-network delay of 5 ms or less, a significant increase in spectrum efficiency and a network architecture designed to simplify network operation and reduce cost. To support such high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The modulation technique or the transmission scheme used in LTE is known as Orthogonal Frequency Division Multiplexing (OFDM).
For the next generation mobile communications system such as International Mobile Telecommunications (IMT)-Advanced and/or LTE-Advanced, which is an evolution of LTE, support for bandwidths of up to 100 MHz is being discussed. LTE-Advanced can be viewed as a future release of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-Advanced can be deployed in spectrum already occupied by LTE. In both LTE and LTE-Advanced radio base stations, also referred to as eNBs or eNodeBs, multiple antennas with precoding/beamforming technology can be adopted to provide high data rates to the user equipments (UEs). Thus, LTE and LTE-Advanced (LTE-A) are examples of Multiple-Input, Multiple-Output (MIMO) radio systems. Another example of a MIMO based system is the Worldwide Interoperability for Microwave Access (WiMAX) system.
In LTE-A, also referred to as 3GPP Release 10 or Rel-10, up to 8 UE-specific reference signals, also referred to as demodulation reference signals (DM-RS), are introduced for the purpose of channel demodulation. A reference signal is a known signal which is inserted at predetermined positions in the OFDM time-frequency grid. The presence of this known signal allows the UE to estimate the downlink channel so that it may carry out coherent channel demodulation. Thus, each downlink antenna port transmits one DM-RS, which is specific to that antenna port as well as to the UE that the transmission is directed to. Thus far, the DM-RS pattern with normal cyclic prefix (CP) supporting up to rank 8 has been specified. A cyclic prefix is a guard interval which is prepended to each OFDM symbol to reduce inter-symbol interference.
The DM-RS signals are transmitted according to a predefined pattern in time and frequency, so that the UE knows where to find the signals. FIG. 1 shows a DM-RS pattern with normal cyclic prefix (CP), supporting up to rank 8. The expression “rank”, or transmission rank, refers to the number of independent data streams, or spatial layers, which may be reliably transmitted over a wireless channel. In the present context, the rank may be interpreted as the maximum number of transmit antenna ports that are supported.
A brief review of the LTE downlink physical resource structure will be helpful. In OFDM systems such as the LTE, the available physical resources are divided into a time and frequency grid. The time is divided into subframes, each comprising a number of OFDM symbols. In LTE and LTE Advanced, a subframe is 1 ms in length, divided into two time slots of 0.5 ms each. For normal cyclic prefix (CP) length, the number of OFDM symbols per subframe is 14, which means that time is quantized into 14 symbols during a subframe. For extended cyclic prefix length, there are 12 OFDM symbols per subframe. Frequency corresponds to subcarriers in the OFDM symbols, and the number of subcarriers varies depending on the system bandwidth used. Each box within the time-frequency grid represents a single subcarrier for one symbol period, and is referred to as a resource element (RE). The smallest schedulable unit of resource elements is called a physical resource block (PRB), or simply a resource block (RB). In LTE and LTE-A, a PRB spans 12 subcarriers and 0.5 ms, i.e. 7 or 6 OFDM symbols depending on cyclic prefix length. The PRBs are allocated in pairs in the time domain. Thus, an LTE subframe of 1 ms comprises two PRBs.
There is also a special type of LTE subframe, composed of three fields: Downlink Pilot Timeslot (DwPTS), Guard Period (GP), and Uplink Pilot Timeslot (UpPTS). This special subframe is used for downlink-to-uplink switching in TDD mode. The duration of the GP field is varied depending on how long it takes the UE to switch between receiving and sending, and also on the signal propagation time from the base station to the UE. The DwPTS field carries synchronization and user data, as well as the downlink control channel for transmitting scheduling and control information. Since the total subframe duration is fixed at 1 ms, the duration of the DwPTS and UpPTS fields are adjusted based on the duration of the GP field.
FIG. 1 shows a time-frequency grid for a normal LTE subframe. Each row in the grid represents a subcarrier, and each column represents an OFDM symbol. The grid covers two LTE time slots, as explained above. The DM-RS pattern of FIG. 1 supports a total of 8 DM-RS antenna ports. The pattern exhibits a DM-RS overhead of 12 REs per layer; that is, each antenna port will use 12 REs for transmitting the DM-RS signals. The 8 DM-RS antenna ports are separated by a combination of code division multiplexing (CDM) and frequency division multiplexing (FDM). It should be noted that the term “antenna port” is used rather than “antenna”, to emphasize that what is referred to does not necessarily correspond to a single physical antenna.
Up to two CDM groups are reserved for DM-RS, where each CDM group consists of 12 REs per PRB pair. A CDM group is a group of REs used for multiplexing reference signals from a number of antenna ports using code division multiplexing. In FIG. 1, the squares marked “1” form one CDM group, and the squares marked “2” form another CDM group. Each CDM group supports a maximum of four layers, i.e. a maximum of four antenna ports. The two CDM groups are multiplexed by FDM; in other words, the REs belonging to the first and second CDM groups are transmitted on different subcarriers.
As seen, there are two CDM clusters, one in each time slot. Furthermore, each CDM group comprises three CDM subgroups. Each CDM subgroup comprises 4 REs in the time domain, and in each CDM subgroup, up to four DM-RS antenna ports may be multiplexed. The REs within each subgroup share the same subcarrier in the frequency domain. For example, the four squares marked “1” in the top row of the time-frequency grid in FIG. 1 form one CDM subgroup of the CDM group 1. The REs of the subgroup being on the same row indicates that they are carried by the same subcarrier. It is also seen that different CDM subgroups are on different rows in the figure indicating that the REs of different CDM subgroups are carried on different subcarriers.
The multiplexing of reference signals within a CDM subgroup is accomplished by applying orthogonal cover codes (OCC) across the time domain. An OCC is a set of codes where all codes in the set have zero cross-correlation. Thus, two signals encoded with two different codes from the set will not interfere with one another. An example of an OCC is a Walsh code. Walsh codes are defined using a Walsh matrix of length N, i.e. having N columns. Each row in the Walsh matrix is one length-N Walsh code. Although Walsh codes will be used in this disclosure to exemplify the invention, it should be understood that any OCC may be used.
Each antenna port transmits one reference signal within the CDM subgroup, by applying an OCC to the signal. If four antenna ports are multiplexed within a CDM subgroup, each of the four antenna ports applies a code of the OCC corresponding to the CDM subgroup. As a way of an explanation, the following is example provided. A length-4 OCC for a CDM subgroup may be visualized as a 4×4 matrix with each row of the matrix representing a code of the OCC applied by a corresponding antenna port. This allows the reference signals to be separated and decoded on the receiver side. Note that for each CDM group, a different 4×4 matrix is used.
At the UE side, per port channel estimation is performed by using the proper OCC for each of the CDM subgroups. That is, each DM-RS signal is decoded using the corresponding OCC that was used to encode the signal. A different length OCC is applied for channel estimation depending on how many layers are multiplexed in one CDM group. Two exemplary cases with two and four layers, respectively, will now be described with reference to FIGS. 2 and 3.
When two layers are multiplexed in one CDM group, a length-2 OCC can be used for the CDM cluster in the first and second time slots respectively, as shown in FIG. 2. This means that the Doppler impact introduced by mobility can be well captured by weighting the two CDM clusters. When the mobility of the UE is high, the time domain channel is likely to vary fast in time. This means that the channel is likely to vary between the first and second time slots in the subframe. Since applying the length-2 OCC means that code spreading is processed in each time slot, the Doppler impact between two slots can be well compensated for by weighting the two slots with proper coefficients that reflect realistic channel conditions.
When more than two layers are multiplexed in one CDM group, a length-4 OCC has to be used across both CDM clusters in one subframe, as illustrated in FIG. 3. Length-4 OCC is typically used for high rank cases. At the UE side, one common strategy for performing DM-RS based channel estimation is to apply a 2×1D filter method per PRB, i.e. first a frequency domain filter and then a time domain filter. The basic principle is shown in FIG. 4. Frequency domain filtering and time domain filtering are performed based on respective inputs of delay spread, Doppler, and received signals. In general, applying the frequency domain filter has been found to be very processing intensive and thus requires a much longer processing time than the time domain filter. Thus, to a significant extent, the time required by the frequency domain filter becomes a bottleneck to speed up the processing on channel estimation and further detection, and this may impact overall detection latency.
When performing channel estimation with a length-2 OCC, as shown in FIG. 2, a slot-by-slot channel estimation can be exploited. In other words, channel estimation based on the signals received in the first slot can be performed before the reception of the whole subframe. The reason for this is that a reference signal is transmitted in two consecutive REs, which are in the same time slot, and all the information required to decode the reference signal is available within that single time slot. This allows the processing required by the frequency domain filter in the first time slot to be commenced before the reference signals in the second time slot are received. This can result in a low latency channel estimator.
However, in Rel-10, a length-4 OCC is used to support multiplexing of up to four layers in each CDM group, as explained above. When performing channel estimation with the length-4 OCC as shown in FIG. 3, the code de-spreading cannot be performed until the whole subframe is received. This is because the reference signal is spread across four REs, which are distributed across two time slots. Thus, in the conventional scheme, channel estimation cannot be performed until signals in both time slots are received. In other words, processing of the first time slot signals cannot commence until immediately after receiving the signals. Thus, additional time will be required, particularly by the frequency domain filter. Consequently, a low latency channel estimator is not suitable for the length-4 OCC for the conventional scheme. In addition, in case of the length-4 OCC, the Doppler impact cannot be well compensated since code de-spreading needs to be considered in both time slots.